This invention relates to a conductor for an electrical power cable and, more specifically, to a large-size conductor for an electrical power cable and a method for manufacturing the same.
Accompanying the remarkable increase in electrical power consumption, the amount of power transmitted has been increasing steadily. With such an increase in power transmission capacity, large-size conductors for power cables have come into use. Recently, conductors with a cross-sectional area of more than 2,000 mm.sup.2, especially, 5,000 to 6,000 mm.sup.2, have been put to practical use.
These large-size conductors, however, are subject to a significant AC loss due to the skin effect. Namely, the increase of the AC resistance due to the skin effect suppresses the increase of the transmission capacity. In order to reduce such AC loss, so-called multi-segmental conductors have been developed. The multi-segmental conductor may be obtained by preparing a small-size segment formed of a shaped-stranded conductor, applying the insulation over the segment, and laying up several such small-size stranded segments into a large-size conductor. Also developed has been an insulating-film-coated stranded conductor in which each strand is covered with an insulating film.
FIG. 1 shows the skin effect coefficient characteristics of three conductors of different types with respect to the cross-sectional areas thereof. In FIG. 1, a characteristic curve A represents the case of an insulating-film-coated stranded conductor, while curves B and C represent cases of an oil-filled cable conductor and a pipe-type-oil-filled cable conductor, respectively. As is evident from FIG. 1, the insulating-film-coated stranded conductor is the lowest among the others in the coefficient of the skin effect for every cross-sectional area, and also in the increasing rate of the coefficient of the skin effect relative to the increase in the cross-sectional area of the conductor. Namely, the larger the cross-sectional area becomes, the more favorable the insulating-film-coated stranded conductor becomes as compared with the other types.
The enamel coating method has been generally used for the insulation of a strand. This enamel coating method, however, has the drawback of being expensive. Also available is a method of forming a surface oxide film on a strand by oxidizing the surface of every stand. In this method, each strand is individually immersed in oxidizing liquid to form an oxide film on the surface of the strand, for example. A plurality of such strands, each covered with an oxide film, are stranded to form a conductor for a cable. In this case, however, the strands already covered with the oxide films are stranded by means of an external force which causes a relatively large frictional force to occur between the strands in the course of stranding, thereby exfoliating the oxide films on the surfaces of the strands.
Furthermore, there is a method of immersing a stranded conductor in oxidizing liquid to oxidize the surface of each strand. In such a method, however, there is a drawback in that the strands are stranded tight at a stage where the conductor is immersed in the liquid, so that the oxidizing liquid will not be able to penetrate deep into the gap between the strands of the immersed conductor, thus oxidizing only the exposed surfaces of the strands at the superficial portions of the strands.
In addition to the coefficient of the skin effect, withstanding voltage and minimum ratio of winding are important factors for the conductor in an electrical power cable. Here, the withstanding voltage is the voltage over which the electrical insulation between two strands with surface insulation films in contact with each other is broken when the voltage is applied therebetween. The minimum ratio of winding is the ratio of the diameter of a mandrel to the diameter of the strand wound on the mandrel, over which the insulation film formed on the strand is exfoliated.
It is desirable for the conductor of an electrical power cable to have good characteristics in the coefficient of the skin effect, the withstanding voltage characteristic, and the minimum ratio of winding.